Add material to text's bevel?
I figured it out: Go into Edit Mode, select some of the bevel's Faces, Click Select → Select Similar → Face Regions (this will select the text's bevel), create a new Vertex Group and assign these selected Faces to it, create a new Material Slot and assign these selected Faces (from the Vertex Group) to it, add a new Material to this new Material Slot and then you're done!